The present invention relates to a cutting method to be applied to producing helical springs by cold-forming a thick high-strength wire in a helical coil and cutting the wire at the end coil of the helical coil to seperate the helical coil, namely, a helical spring from the wire.
The company to which the inventor of the present invention belongs has previously developed a technique for producing highly cold-formable high-strength spring wires having a tensile strength of 1471 N/mm.sup.2 or above, in which a wire is hardened by heating the wire at a high rate through induction heating or the like and quenching the heated wire, then the hardened wire is heated through induction heating or the like at a high rate for a short time of 60 sec. or less at a predetermined temperature in the range of 300.degree. to 600.degree. C., and then the hardened and heated wire is cooled while the wire is fed continuously to the hardening process, and a technique for producing a high-strength spring having very excellent spring properties, in which the high-strength spring wire porduced by the former technique is cold-coiled in a helical spring, and then the helical spring is held at a predetermined temperature in the range of 300.degree. to 500.degree. C. for a predetermined time in an electric furnace or the like. These techniques are disclosed in U.S. Pat. Nos. 4,407,683 and 4,336,081, respectively.
Thick high-strength spring wires manufactured by the technique according the invention disclosed in U.S. Pat. No. 4,407,683 are, in most cases, those having a diameter in the range of 8 to 16 mm and a tensile strength in the range of 1765 to 2157 N/mm.sup.2 for the suspension springs of vehicles. In manufacturing a helical spring according to U.S. Pat. No. 4,336,081 by using the thick high-strength spring wire manufactured through processes according to U.S. Pat. No. 4,407,683, the spring wire is cold-coiled in a helical coil and the spring wire is cut at the end coil of the helical coil to provide a helical spring. These processes of the prior art for cold-coiling the spring wire and cutting the spring wire will be described briefely hereinafter with reference to FIG. 1.
In FIG. 1, there are shown an supply stand 1 and a coiling machine CM. The coiling machine CM comprises, as principal components, feed rollers 2, a wire guide 3, coiling rolls 4a and 4b, a pitch tool 5, a cutting mandrel 6 and a cutter 7. A coil of a thick high-strength wire W is supported on the supply stand 1. The thick high-strength wire W is drawn out from the unwinding stand 1 by the feed rollers 2 passes through the wire guide 3 and engages the coiling rolls 4a and 4b sequentially. The coiling rolls 4a and 4b deflect the wire W from the original course of advancement so that the wire W is wound around the cutting mandrel 6, while the pitch tool 5 regulates the pitch of coils of the wire W so that the coils are formed at a predetermined pitch. Thus, a predetermined number of coils are formed on the cutting mandrel 6. Upon the completion of forming a predetermined number of coils of the wire W, the wire feeding operation is interrupted temporarily. While the wire feeding opration is interruped, the wire W is cut at the end coil by nipping a position in the end coil between the stationary cutting mandrel 6 and the vertically movable cutter 7 to separate the coils of the wire W from the wire W. Thus a helical spring is provided.
Cutting the wire W at the end coil requires a very large shearing force when the wire W has a large diameter and a high strength. For example, the shearing force required for cutting a wire having a diameter of 14.0 mm and a tensile strength of 2,010 N/mm.sup.2 is ##EQU1##
Since the cutting mandrel 6 and the cutter 7 are required to apply such a large force to a 14.00 mm diameter wire, the edge of the cutter 7 is nicked or the cutting mandrel 6 and the cutter 7 are broken very often, even if the material and hardness of the cutting mandrel 6 and the cutter 7 are selected properly, and hence the service life of the cutting mandrel 6 and the cutter 7 is very short. Therefore, the ratio of the maintenance cost including the costs of those members to the helical spring forming cost is very large.
Since cold-coiling a thick wire is difficult, it is common knowledge in manufacturing helical springs to hot-coil a wire in a helical coil, to cut the helical coil from the wire and to subject the helical coil to heat treatment to produce a helical spring having desired mechanical properties. The invention disclosed in U.S. Pat. No. 4,407,683 constituted a breakthrough and overcame the difficulty in cold-forming helical springs. However, the invention brought about the above-mentioned new difficulty in cutting the end coil of the helical coil.
Cutting a thick high-strength wire at the end coil in manufacturing helical springs according to the above-mentioned prior invention has never been dreamt of. Accordingly, there is no prior art which is analogous to the present invention within the knowledge of the inventor of the present invention.